As dimensions of semiconductor devices shrink to improve performance, the need to control film thickness to thinner and thinner uniform dimensions increases. Thickness uniformity is important as dimensions shrink to minimize overlap with adjacent devices. Improvement in uniformity is especially important in current technologies utilizing shallow (0.25 micron) source/drain diffusions. Because shallow diffusions can be obtained by implanting through an oxide, any non-uniformity in that oxide thickness would result in non-uniform source/drain diffusion depths, degrading device performance. Improved thickness uniformity is also important during etching to minimize over-etching which is directly dependent on film uniformity.
Most films deposited by CVD or ALD for semiconductor applications are grown using conventional bubbler technology with a carrier gas bubbled through a neat (i.e., without solvent) precursor at an elevated temperature, relying on the vapor pressure of the precursor to be constant in order to deliver a uniform precursor flux to the film. However, because vapor pressure is directly related to temperature, conventional bubbler technology suffers from the disadvantage of needing to maintain a bubbler temperature with minimal variation during a run or from run to run. Fluctuations in precursor flux are known to result in variable film growth rates. Solid compounds are known to sinter and change surface area over time, resulting in non- uniformity in film growth rates from run to run. Sintering is not a problem for liquid precursors, but over time the liquid precursors may degrade from the thermal cycling and thermal load placed on the precursor. In addition, at elevated temperatures, decomposition processes are accelerated. Elevated temperatures and thermal cycling of a precursor during vaporization in a conventional bubbler may contribute to premature degradation of the precursor over time. Precursors may change their chemical state by ligand rearrangement, cluster formation, or oxidation. Precursors may react with water or oxygen inadvertently introduced into the bubbler through inadequately purified carrier gases bubbled through the precursor, air leaks, or water and oxygen adsorbed on the bubbler walls.
Examples of precursors which are typically used in conventional bubbler technology and suffer from the disadvantages listed above include hydrides, alkyls, alkenyls, cycloalkenyls, aryls, alkynes, carbonyls, amides, phosphides, nitrates, halides, alkoxides, siloxides, and silyls. Thermally unstable hydrides such as alkylamine alanes are especially attractive for CVD and ALD because their high reactivity usually translates into low thermal processing temperatures, and reduced impurity incorporation. Unfortunately, the alkylamine alanes are notoriously unstable during storage, transport and vaporization leading to poor reproducibility in the resultant films. Alkylamine alanes such as trimethylamine alane, triethylamine alane and diethylmethylamine alane are known to decompose at temperatures greater than 40° C. during storage, and during transportation to the CVD reactor. Care must be taken to store the precursors at room temperature or less to minimize decomposition. Transport and vaporization temperatures are thus limited by the thermal decomposition of the precursor, See, for example, Dario M. Frigo, and Gerbrand J. M. van Eijden, Chemistry of Materials, 1994, 6, 190–195 and C. E. Chryssou and C. W. Pitt, Applied Physics A Materials Science and Processing, vol. 65, 1997, 469–475.
Another example of thermally unstable precursors are Cu(I) compounds such as (cyclopentadienyl)Cu(PEt3) which is known to decompose with loss of PEt3 at temperatures as low as 70° C. Other examples include alkyls such as trimethylindium and triethylindium. Triethylindium is a liquid and is known to decompose at room temperature in the bubbler. Trimethylindium is a solid at room temperature and over time a change in the effective vapor pressure is observed resulting in undesirable non- uniformities and irreproducibility of growth results, See, G. B. Stringfellow, Organometallic Vapor-Phase Epitaxy: Theory and Practice (San Diego, Calif.: Academic Press, 1989).
Other examples include alkoxides which are known to change their chemical state by ligand rearrangement, hydrolysis, olgiomerization, ring formation, cluster formation, and/or oxidation over time. At the elevated temperatures encountered in conventional bubbler technology, these decomposition processes are accelerated. In addition, alkoxides are particularly sensitive to water and oxygen impurities which may be inadvertently introduced into the bubbler through inadequately purified carrier gases bubbled through the precursor, air leaks, or water and oxygen adsorbed on the bubbler walls. Hydrolysis reactions can occur and these reactions are accelerated at elevated temperatures which are commonly encountered in conventional bubbler technology. Alkoxides also may exist in a number of isomeric forms which interconvert over time resulting in a variable vapor pressure. For example, aluminum isopropoxide exists in a number of isomeric forms with slow interconversion rates between the isomers. The vapor pressures of these isomers vary widely, making it difficult to control the deposition rate of Al2O3 grown from this compound using conventional bubbler technology, See R. G. Gordon, K. Kramer, X. Liu, MRS Symp Proc. Vol. 446, 1997, p. 383.
Other examples include amides which behave similiarly to alkoxides, being prone to ligand rearrangement, hydrolysis, oxidation, olgiomerization, ring formation and existing in several interconvertable isomeric forms resulting in irreproducible vapor pressures over time. Other examples include anhydrous metal nitrates such as titanium nitrate, zirconium nitrate and gallium nitrate. These complexes are air and water sensitive and are known to decompose at temperatures around 100° C. The metal oxo-nitrates such as VO(NO3)3 and CrO2(NO3)2 are light sensitive, in addition to being air and water sensitive, and should be stored at 0° C. This is disclosed, for example, in D. G. Colombo, D. C. Gilmer, V. G. Young, S. A. Campbell and W. L. Gladfelter Chem. Vap. Dep. 1998, 4, No. 6, 1998 P. 220.
The use of β-diketonate containing precursors dissolved in solution for CVD growth has been described previously. U.S. Pat. Nos. 5,204,314, 5,225,561, 5,280,012, 5,453,494, and 5,919,522 disclose the growth of Ca, Sr, or Ba containing films using a solution containing a Ca, Sr, or Ba complex bound to at least one β-diketonate ligand, or β-diketonate derivative. U.S. Pat. No. 5,555,154 discloses the growth of PbZrTiO3 by chemical vapor deposition using a solution containing Pb, Zr and Ti dipivaloylmethanates in tetrahydrofuran. U.S. Pat. Nos. 5,677,002 and 5,679,815 disclose the growth of tantalum and niobium containing films using absolution of Nb and Ta bound to a least one β-diketonate ligand or β-diketonate derivative. U.S. Pat. No. 5,698,022 teaches precursor composition useful for chemical vapor deposition of lanthanide metal/phosphorus oxide films, comprising a precursor compound comprised of a lanthanide metal β-diketonate and a phosphorus containing ligand in a solvent. U.S. Pat. No. 5,783,716 teaches the growth of Pt by CVD using a solution containing a Pt complex bound to a least one β-diketonate ligand, or β-diketonate derivative. U.S. Pat. No. 5,820,664 teaches a metal source reagent liquid solution useful for chemical vapor deposition comprising a metal coordination complex including a metal coordinatively bound to a least one β-diketonate ligand or β-diketonate derivative. U.S. Pat. No. 5,900,279 teaches a solution comprised of a β-diketonate containing precursors dissolved in one of the ligands of the complex. U.S. Pat. No. 5,916,359 discloses the growth of SrBi2Ta2O9, by CVD using a precursor composition consisting of Sr, Bi, Ta β-diketonate containing precursors dissolved in a three component solution of two different C6–C12 alkanes and a glyme-based solvent or polyamine. U.S. Pat. No. 5,980,983 teaches the use of a mixture of metal β-diketonates for deposition of a metal containing film. Despite the numerous disclosures of β-diketonate precusors, β-diketonate-containing precursors are known to have complex decomposition pathways which can lead to incorporation of substantial quantities of carbon or other unwanted impurities into the resultant films.
U.S. Pat. No. 5,900,279 teaches a solution useful for CVD comprised of a metallic organic compound added to a liquid which consists essentially of a ligand of the metallic organic compound. For example, dissolving a M(β-diketonate) in the β-diketone. This reference suffers from the disadvantage of having a huge excess of ligand present during decomposition of the precursor to form the film. The ligand solvent is prone to the same decomposition pathways as the precursor and the precursor decomposition fragments and thus may hinder decomposition of the precursor in the gas phase or on the film surface. Gas phase reactions between the vaporized precursor and partially decomposed precursor and the vaporized ligand solvent and its decomposition by-products are likely and may result in reduced volatility of the precursor, particulate formation in the vaporizer and reactor, and consequently, irreproducible growth rates.
In view of the drawbacks with prior art deposition processes, there is a continued need for developing deposition processes wherein new and improved precursor source mixtures are used for forming thinly deposited layers or films that can be used in various electronic devices.